The invention relates to a method for point rotation sawing of crystalline rod material at off-orientation angles. In another aspect the invention relates to a method for sawing cylindrically shaped bodies transversely of their longitudinal axis wherein the cylindrically shaped bodies contain a crystalline rod with its crystallographic axis at an angle to the cylindrical axis of the encapsulation.
Inasmuch as the invention has particular importance and advantages in its application to the sawing of semiconductor crystals and to slices at off-orientation angles, it will be described in this environment by way of example. However it will be appreciated that the method may be employed with advantage wherever it is necessary to point saw rotating rods at off-orientation angles.
In the commercial production of semiconductor devices, the semiconductor material is initially prepared in relatively large monocrystalline ingots or rods by one or several known methods such as the Czochralski process, float zone, or the like. The ingot usually is of a rod-like configuration and the first step toward its subdivision, ultimately to form wafers utilizing the semiconductor devices is to saw the ingot transversely to thin slices i.e. wafers. This process is usually accomplished at the present time by means of rotary diamond saws which in the simplest form, consists of an inside diameter saw disk surrounding a rod workpiece which is mounted in a stationary position while the inside diameter saw is rotatably driven into contact and through the workpiece.
The use of semiconductor wafers in the electronics industry has become wide-spread for a number of different commercial and technological purposes. These wafers are extremely thin, typically in the order of 0.015 inches or less in thickness for silicon and may range upward in size in diameter of 3 to 5 inches or more. Such wafers are cut from ingot of crystal material which have been grown specifically for the purpose of providing materials to be used in thin wafer form. The uniformity of size and surface of the cut wafers is extremely important; thus the method is needed which can repeatedly cut wafers of desired precise thicknesses having smooth unscarred surfaces. Of importance is a method which will provide such wafers minimizing breakage problems which exist in prior sawing methods.
Presently semiconductor crystal slicing is done by the use of inside diameter cutting saws supplied to a fixed semiconductor rod mounted on the saw exit side in carbon or suitable epoxy materials. The inside diameter sawing of the fixed rod permits the use of a thin circular steel saw plate. As the inside diameter saw proceeds through the cutting path, increasing the saw to crystal rod contact, lengths occur resulting in crystal particles buildup which rubs against the crystal face requiring additional horsepower and resulting in scarred wafer surfaces. Other problems arise from the crystal particle buildup such as flat cuts, irregularly shaped slices and the like. The major force, i.e., energy required in inside diameter slicing or sawing of fixed workpieces is accountable to the buildup of sawed crystal materials along the length of the blade to crystal rod interface. This resulting crystal debris is not readily removed from the interface creating additional friction which results in increasing energy requirements. These inside diameter sawing techniques utilize a thin disk with a comparatively large hole in the center which is tightly stressed outwardly and mounted in a rotating head. The inner rim of the center hole is coated with diamond particles embedded in a nickle matrix and serves as the cutting edge. The semiconductor material to be utilized is drawn into the spinning inner rim where wafers of the materials are sawed. These inside diameter sawing techniques frequently result in the breakage of wafers or splintering from the crystal rod as the inside diameter saw approaches slicing completion and exits from the outside of the rod. The slice actually breaks off the rod before the blade has entirely cut through the rod. The rod being held on a carbon or other satisfactory material mounting block which also functions to hold in place the resulting sawed wafer. Wafer breakage, edge chipping, slant sawing, and scarred or marred wafer surfaces promote low operational yield in the semiconductor industry. Presently used inside diameter sawing techniques also fail generally in sawing other semiconductor materials such as sapphire. Thin inside diameter blade sawing of very hard mineral materials such as sapphire substantially destroys the blade after only a few wafers are sliced due to the hardness and degree of difficulty of slicing a non-rotating rod.
The point sawing of crystal rod materials wherein the semiconductor rods are rotating during the sawing process provides the desirable direction of these problems resulting in a minimization of waste ingot material and improved wafer quality. The improvements are achieved while the method also provides an utmost important capability of repeated slicing utilizing the sawing and/or lapping means wherein the sliced wafers exhibit uniformity of thickness, high surface quality resulting from the sawing method which provides efficient manufacturing operations. Point sawing of rotating crystal rod methods do not completely satisfy needs of the semiconductor market since the crystallographic orientation of the produced wafers varies with utilization by the industry. Particularly in the use of silicon crystal material, the crystallographic orientation desired by users of the produced wafers may vary from about 0 to about 4 degrees from the (1, 1, 1) or (1, 0, 0) crystallographic plane. Wafers obtained by present methods of rotating point sawing do not have, for example, the critical 0 to 4 degrees off-orientation angle which is necessary for use in certain semiconductor devices.